[Migrants are pictured at a Turkish coastguard station after a failed attempt at crossing to the Greek island of Lesbos, in the Turkish coastal town of Dikili]BUDAPEST (Reuters) - Hungary plans to hold a referendum on future European Union quotas for resettling migrants in September or early October, Prime Minister Viktor Orban's cabinet chief said on Tuesday. Antal Rogan said that after parliament gives the plan the green light, President Janos Ader will have to set the date for the referendum on mandatory migrant quotas, which Rogan said was an "issue of national sovereignty". (Reporting by Krisztina Than and Gergely Szakacs)